The use of the time domain electromagnetic method to delineate saline groundwater in granular and carbonate aquifers and to evaluate their porosity

Abstract

The use of surface geoelectric (GE)/geoelectromagnetic (GEM) methods to determine the porosity of freshwater saturated aquifers using Archie's equation leads in many cases to significant inaccuracy. This is caused by the uncertainties in determining both fluid and bulk resistivities required for calculating porosity of a rock sequence in the case of a known rock type. In order to solve the problem in coastal and other saline-water-intruded aquifers, it is proposed herein to apply the time domain electromagnetic (TDEM) method to detect first the fresh/saline water interface and then to determine the fluid and bulk resistivities of the saline water saturated portion of the aquifer. It is shown that calculating porosity of this portion of the aquifer is much more accurate than that of the freshwater saturated part. Since the porosity of the aquifer is generally invariant with regard to the location of the interface, the proposed method in many cases is applicable to the entire aquifer. The method was tested in both granular and carbonate coastal aquifers of Israel saturated with normal seawater as well as with Dead Sea concentrated brines. The cases of granular coastal aquifers exhibit a good agreement between measured porosities and those calculated using TDEM results. In the case of carbonate aquifers, the situation is more complicated due to their heterogeneity and much wider range of porosities stemming from their karstic and fractured nature.